Immunoassays utilizing antigen-antibody reactions have been extensively performed to determine slight amounts of substances in samples in the field of clinical examination. Among these, latex immunoturbidimetry with particulate latices carrying antibodies (hereinafter also referred to as sensitized particulate latices) has been extensively used in laboratories because the latex immunoturbidimetry can be achieved by a simple operation for a short time. In latex immunoturbidimetry, the amount of an antigen or an antibody in a sample is determined through optical detection of a change in absorbance caused by agglutination of a sensitized particulate latex during formation of immune complexes. This change in absorbance is based on an apparent change in particle size caused by agglutination of the sensitized particulate latex.
As described in Patent Document 1, a polystyrene particulate latex mainly composed of polystyrene has been used in latex immunoturbidimetry because of ease in immobilization (sensitization) of antigens or antibodies specifically reactive with their target substances, relatively low cost, and easy control of the polymerization reaction of these particles. Regardless of such an advantage as physical adsorption (sensitization) of antigens or antibodies, the polystyrene particulate latex can also adsorb non-target proteins in samples. This adsorption of non-target proteins may cause so-called non-specific reactions, i.e., agglutination reactions of sensitized particulate latex not caused by a specific antigen-antibody reaction. The non-specific reactions should be prevented.
According to Patent Document 1, a particulate latex sensitized with an antigen or an antibody is blocked with bovine serum albumin (BSA) to prevent the non-specific reactions. Unfortunately, such blocking is still insufficient, and generates high background values. Accordingly, this measure has a severe challenge in preparing of reagents which enables highly sensitive measurement.
Patent Document 2 discloses preparation of polymer particles as carrier particles for diagnostic reagents. The polymer particles are prepared by aqueous copolymerization of styrene, a compound represented by the formula (101), and a salt of styrenesulfonic acid in the presence of a water-soluble radical polymerization initiator.CH2═CR3—COO(CH2CH2O)x(CH(CH3)CH2O)y(CH2CH2O)zR4  (101)where R3 represents H or CH3; R4 represents H or CH3; x, y, and z each represent 0 or an integer of 100 or less and satisfy the relation 1≦x+y+z≦100.
According to Patent Document 2, the resulting polymer particles pose less non-specific reaction. Unfortunately, such polymer particles containing a high content of compound represented by the formula (101) reduce a non-specific reaction, while barely adsorb antigens or antibodies physically onto their surfaces and cannot function as carrier particles for a diagnostic reagent. Furthermore, Patent Document 2 does not teach or suggest any compound for the preparation of the carrier particles except for styrene, the compound represented by the formula (101), the salt of styrenesulfonic acid, and the water-soluble radical polymerization initiator.
Patent Documents 3 and 4 each disclose an immunoassay reagent using as an agglutination accelerator a polymer containing as a main component monomer a compound represented by the formula (102):CH2═CR6—CO—X—R5—O(PO2)OR4—N—(R1)(R2)(R3)  (102)where R1 to R3 each independently represent a hydrogen atom or an alkyl group optionally having a hydroxyl group; R4 represents an alkylene group; R5 represents an alkylene group optionally having a substituent and/or an oxygen atom in the chain; R6 represents a hydrogen atom or a methyl group; X represents an oxygen atom or an —NH— group.
According to these documents, the polymer may be a copolymer which preferably includes 20% or more structural units represented by the formula (102). The copolymer may contain a styrene derivative as a structural unit other than the structural unit represented by the formula (102). Unfortunately, these documents do not teach or suggest that the copolymer is present in a reaction solution as a linear polymer in a free state and is used in a form of a particulate latex.
According to Patent Document 4, addition of an agglutination accelerator, such as polyethylene glycol, to a reaction solution results in non-specific turbidity due to salting out to increase a blank value whereas such an agglutination accelerator including a polymer containing the compound represented by the formula (102) as a main-component monomer barely causes non-specific turbidity due to salting out. Unfortunately, Patent Document 4 does not teach or suggest that the polymer can prevent excess agglutination of dissociation samples that make non-specific agglutination reactions.